As an example of modern communication spread spectrum signals, the following is a description of known signals broadcast by Global Positioning System (GPS), which includes 24 satellites orbiting the Earth.
In GPS, each satellite broadcasts carrier signals on the same frequencies. Navigational data is spread by a clear acquisition (C/A) code and modulated on the L1 (1575.42 MHZ) in-phase channel. The navigational data is further spread by a P(Y) code and modulated on both the L1 quadrature and L2 (1227.60 MHZ) in-phase channels.
As both military and civilian requirements for GPS change, a condition which will likely continue into the 21st century, operational modifications become increasingly necessary. Many of these modifications revolve around the waveform structure of GPS.
One of the requirements that has changed since awarding of a contract for the next generation GPS satellites (Block IIF) is a requirement for an additional military signal on the same L band carrier as the C/A and P(Y) signals. An Interplex modulation technique is used to combine the three signals.
Interplex modulation is a technique for combining three or more signals to generate a constant envelope composite signal. A constant envelope composite signal is highly desirable, because it allows a highly efficient power amplifier to be utilized.
As an example, for three signals, S1, S2 and S3, Interplex modulation allows combining these signals into a phase modulated composite signal that maintains a constant envelope, thereby allowing the use of a high power amplifier without signal distortion. Taking these three signals, S1, S2 and S3, an Interplex modulator generates a composite signal that includes three desired components plus an unwanted cross-product.
Signals generated by Interplex modulation are single-carrier, double sideband signals, which have a serious shortcoming and restriction. It is often desired to generate, amplify, and transmit spread-spectrum signals having multiple carriers, with each carrier modulated by one or more spreading codes.
The present invention addresses this shortcoming by providing a composite signal that includes multiple carriers, with each carrier modulated by one or more spreading codes. Moreover, the present invention provides this composite signal as a constant-envelope, single-sideband signal, thereby resulting in systems with high power and spectral efficiency.